The loudness of sound is determined by the

Explanation:
Loudness of sound depends on intensity of sound waves. It can be determined by the given formula,
L = 10log10 II0 ( where I is the intensity of sound waves and L is the loudness)
Since, Intensity of the sound waves depends on its amplitude, therefore, Loudness depends on amplitude of vibration.

Final answer:
Hence the correct option is A, i.e. amplitude of vibration.